Drive controls for magnetic recorder-reproducer



Jan. 14, 1958 Original Filed Aug. 11,

J. R. SORRELLS DRIVE CONTROLS FOR MAGNETIC RECORDER-REPRODUCER 6Sheets-Sheet 1 WW [0 0. p rs a F use & Q /02 0 Q o o o o W U [I06 I 1040 i Q 6 r C) Q o I09 hm H 1 H H U H (I w J\ b J\ a D n J 0 j INVENTORJohn K. for-rails ATTORNEY Jan. 14, 1958 SORRELLS 2,819,940

DRIVE CONTROLS FOR MAGNETIC RECORDER-REPRODUCER Original Filed Aug. 11,1954 6 Sheets-Sheet 2 SPROCK ET PULSES 204 OUTPUT Raw/V6 SPROC/(ETSIGN/ILL; AMP & SCHMITTfi GATE PULSE DELAY TRIGGER PULSE H90 (H63)CIRCUIT TUBE SHAPER MULT/V. AME amps/z 5,721,522] 202a 1 2224 207 208209 DE/i/AY M111. u DIFFERENT. 205

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DRIVE CONTROLS FOR MAGNETIC RECORDER-REPRODUCER Original Filed Aug. 11,1954 6 Sheets-Sheet 5 7D SCHMITT 200x 4B SPROCKFT CH4. 4/7220/ TMILUAMPS 20"; BACK VOLTS 10- J okwaga vous l aloe 2' --200 300MICRO/M025 CONDUCT/YE STE/P i 0N REVERSE END 400 OF TAPE 1 500 )v0/vMAGNET/c FACE OF TAPE CONDbCT/VE 5TIP 0N FORWARD END OF TIM-"6 INVENTORjohn R Sorrel/5 BY %W 'ATTORNEY Jan. 14, 1958 J. R. SORRELLS DRIVECONTROLS FOR MAGNETIC RECORDER-REPRODUCER Original Filed Aug. 11, 1954 6Sheets-Sheet 4 (a) W y r 7 (a) III I (1)) W,- fi. E T

no 1/0 31 H1, L I- (c) g k ATTORNEY Jan. 14, 1958 J. R. SORRELLS DRIVECONTROLS FOR MAGNETIC RECORDER-REPRODUCER 6 Sheets-Sheet 6 OriginalFiled Aug. 11, 1954 SQ RE SE m MmQwEQ QQQRQQK n mEw Nu NI n, m?

INVENTOR fo/m R Sorrel/s ATTORNEY DRIVE CONTRCLS FOR MAGNETICRECORDER-REPRODUCER John R. Sorrells, Silver Spring, Md., assignor tothe United States of America as represented by the Secretary of CommerceOriginal application August 11, 1954, Serial No. 449,286.

Divided and this application March 9, 1956, Serial No- 570,650

3 Claims. (Cl. 346-74) This invention relates to a memory orinformationstorage device such as is commonly employed with largescaleautomatic computers where it is desired to store coded information sothat it may be withdrawn when needed and rewritten, or altered asnecessary. The information stored in such memory may represent eitherinstructions for further computation or numbers to be operated on, butin either case the information is usually stored as a series of codedbinary digits.

The present invention is a division of application Serial No. 449,286,Multichannel Tape System of Storage, filed August 11, 1954, by John R.Sorrells.

The system contemplated in this disclosure is one which usesmultichannel magnetic tape as a storage medium together with a system oftransducer elements by which access to any desired part of the storedinformation may readily be obtained, while additional circuits areprovided to enable the same transducers to store, or change the natureof the information already stored in the tape.

The present invention employs principles which are commonly known andemployed in existing storage systems, such as represented, for example,in the patent to Cohen et al., Serial No. 2,540,654, issued on February6, 1951. In such systems, in which a magnetic drum is employed as thestoring medium, a synchronizing track is employed, which acts as acontrol for an additional number of collateral tracks on whichinformation signals are stored. The synchronizing track provides a meanswhereby any particular area on the collateral in formation-gatheringtracks can be picked out with precise accuracy. However, in devices suchas that represented in the Cohen patent, which employ a magnetic drum asa storage medium, suflicient physical space is provided by virtue of thesize of the cylindrical drum employed, whereby various transducerscommon to each track can be adequately spaced and staggered with respectto one another so that the danger of cross-talk between adjacenttransducers is avoided. In the case of a relatively thin band ofmagnetic tape which is commercially available in half-inch widths,however, if it is desired to employ a plurality ofinformation-magnetizable tracks together with a synchronizing-channeltrack parallel to one another along the length of the tape, it isobvious that space limitations necessitate extremely close spacing ofthe transducer heads in order to cover the track, and considerableinteraction or cross-talk between adjacent transducers results. Thepresent invention contemplates the use of a plurality of informationchannels in a standard narrow magnetic tape in a manner which avoids thedeleterious efiects consequent to cross-talk and yet permits very closespacing of the transducer heads in order to utilize the maximum surfaceof the tape.

It is therefore an object of this invention to provide aninformation-storage system, employing commercially available magnetictape of relatively narrow width, which enables the use of a plurality ofinformation channels together with a synchronizing channel.

States Patent-O Computer Magnetic Tape Mechanism, manufactured:

Another object of this invention is to provide an information-storagesystem of the type described which provides a means for readily storingand reading information from the magnetic tape but which eliminates theerrors resulting from the presence of cross-talk among the varioustransducers.

A further object of this invention is to provide a control circuit for amagnetic drive mechanism, which in response to only two applied inputdirection signals, will exercise three conditions of control of themovement of the tape.

A still further object of this invention is to provide an automatic endstop for the tape drive mechanism which will prevent over-running of thetape as it approaches either end of the reels.

Further objects will become apparent as the description proceeds. Apreferred embodiment of the invention is illustrated in the accompanyingdrawings, in which:

Fig. 1 shows a standard type of magnetic tape-handling mechanismemployed with this invention;

Fig. 2a is a block diagram of the circuit elements used to store andalter information on the magnetic tape;

Fig. 2b is a block diagram of the circuitry involved in sensing orreading information;

Fig. 3 is a schematic circuit diagram showing the details of thesprocket-channel amplifier comprising the invention;

Figs. 4a-4d illustrate the nature and relation of certail;1 signalsrepresenting the coded information dealt Wit Figs. 5a5c illustrate waveforms explaining the operation of the sprocket-channel amplifier;

Fig. 6 is a curve showing typical characteristics of a germanium diode;

Figs. 7a7c, 8a-8c, 9a9c, and 10a-10c are oscillograms showingcharacteristic wave forms of the signals at various points on thesprocket-channel amplifier;

Figs. Ila-11c and 12a-l2i show the character of the signals in anotherportion of the circuit shown in Fig. 2;

Fig. 13 illustrates the circuitry employed to select the informationchannels on the tape;

Fig. 14a is a circuit diagram showing the improved control circuit forthe tape-handling mechanism shown in Fig. 1;

Fig. 14b is a chart for consideration in connection with Fig. 14a;

Fig. 15 shows the details of the end-stop senser;

Fig. 16 illustrates a desired modification to the tape; and

Figs. 17a through 17e diagrammatically illustrate the equivalent circuitconstruction of various logical elements shown in symbolic form in Figs.2a and 2b.-

Fig. 17a shows the electrical symbol for an and-gate circuit;

Fig. 17b shows a typical electrical circuit for the andgate circuitshown in Fig. 17a;

Fig. 17c shows the electrical symbol for an and-gate plus an inhibitsignal circuit;

Fig. 17d shows the electrical components for the gate circuit shown inFig.

Fig. l7e illustrates another embodiment of a gate circuit. I

This invention employs a standard commercial magnetic tape-handlingsystem, such as shown in Fig. 1, together with a special circuit forrecording and reading information which is illustrated in block diagramform in Figs. 2a and 2b. The unit preferably employed is a by theRaytheon Manufacturing Company.

A Well-known and widely used system for storing information on magnetictape is the single-channel type of storage, which is quite simple andrequires a minimum of equipment for use. This system requires someprovision for interpreting the information received from an externalsource and supplying the proper signals to the recording head, circuitryfor amplifying and interpreting the signals from the reading head, andsome means for erasing the tape. The simplicity of single-channelrecording is a desirable feature, but such system also has some distinctdisadvantages. One objection to this system is that it does not makeeflicient use of the storage capacity of the tape. it, for instance, asingle broad channel is recorded on the tape when it is feasible torecord three narrow channels on the same tape, then only a third of thestorage capacity of the tape will be utilized. With the existingemphasis on etficiency and compactness, such a waste of recordingsurface would be diflicult to justify, provided a more efiicientmultichannel system sacrifices nothing in the way of reliability andflexibility.

A further objection to single-channel recording, which also applies tosome multichannel systems, is the difliculty, or, in some cases, theimpossibility of altering a small specific part of the information onthe tape. This trouble results from insufficient control of the erasingand recording processes; specific information cannot be erased and newinformation recorded in its place without changing adjacent informationwhich is to remain unaltered. In systems where such control is lacking,no attempt is made to selectively alter any information, and as aconsequence the system loses some of its flexibility.

Another disadvantage of single-channel and some mulitchannel storagesystems is that the reliability of the system is greatly dependent uponthe quality and condition of the tape. Tape flaws due to imperfectionsin the magnetic surface are a considerable source of errors unlesscertain precautions are taken. To assure acceptably reliable operation,either the flaws must be removed from the tape or the parts of the tapeWhere flaws occur must be deleted from further use. Although theseprecautionary measures greatly improve the reliability of the system,they often require an undue amount of tape testing, or result in wastedor inetficiently used tape.

The foregoing disadvantages and limitations of singlechannel and sometypes of multichannel storage are pointed out in order to emphasize thefeatures of the multichannel storage system comprising the presentinvention and to serve as a basis for its evaluation. In contrast tosingle-channel systems, the present system makes very efficient use ofthe tape, is capable of selectively altering as little or as much of thestored information as is desired, requires no erasing of the tape priorto correction, and makes possible the elimination of errors due to tapeflaws. These improvements are made possible by using one of theavailable tape channels as a synchronizin or sprocket channel as in thereferred-to Cohen patent, but employing a novel amplification and gatingsystem associated With the closely spaced transducer heads, which makesfeasible the use of closely spaced recording channels in connection withrelatively narrow recording tape. Before going into further details asto how such objectives are achieved, the method for generally storinginformation on magnetic tape will be briefly described.

Information is generally stored on the tape in coded binary form, andsince the binary system of notation requires only 1s and Us to representany given number, use of this system requires any memory cell to becapable of storing or remembering either of two conditions of magneticpolarization in order to store one binary digit. A memory cell is adiscrete area measured lengthwise of the tape which may be polarizedv toa magnetic. state distinguishable from an adjacent cell area. A singlebinary digit appears on the tape then as a small discrete area which ismagnetically saturated in either a positive or a negative direction. Anarea which is magnetized in the positive direction is defined as abinary 1, and an area magnetized in the negative direction is defined asa binary 0. When using a magnetic tape information-storing device inconnection with serial type machines, information can be recorded orread from only one channel of the tape at a time. This means that alldigits in a word or block of information must be stored on a singlechannel of the tape in a sequential order. Typical wave forms involvedin the storage of a sequence of digits are illustrated in Figs. 4a-4d.The type of magnetic recording system illustrated therein is termedreturn-to-zero recording, since the magnetization of the tape alwaysreturns to a zero value between successive digits or bits of informationas shown in Fig. 4b, in which the wave form returns to a zero restinglevel value between occurrences of either a positive or a negativepolarizing pulse. Fig. 4a, illustrates the recording current wave formto pro duce the magnetization of Fig. 415, while Fig. 4c shows thereading voltage obtained by sensing a tape which has been recordedaccording to Fig. 4b. Fig. 4d indicates the digit values represented bythe wave forms of Figs. 4a-4c.

The function of the sprocket channel is to maintain precise control overthe recording or reading of each binary digit in any informationchannel. Precisely timespaced unidirectional pulses are recorded in thesprocket channel before any information is stored in any of theinformation channels, and thereafter an information digit is recordedon, or read from, an information channel only coincidently withdetection of a control pulse in the sprocket channel. The pulses on thesprocket channel then, in effect, determine the exact location of eachcell in an information channel where a digit may be stored, and unlessthe pulses in the sprocket channel purposely altered, the location ofeach storage cell remains thereby fixed for the life of the tape.

This method of permanently and exactly defining the spot where each andevery digit of information is to be stored affords an excellent means ofeliminating errors due to tape flaws. Before a tape is put into use, thesprocket channel is recorded from end to end with precisely spacedsynchronizing pulses. This channel is then read or sensed and, bysuitable circuitry, only a play-back pulse above a certain minimumamplitude is selected to cause a pulse to be recorded in the adjacenttrack or channel. Such channel. will then have pulses stored on it atintervals corresponding to only those points on the sprocket channelwhere there are no flaws, because a sensed signal arising out of a tapeflaw will be below such minimum amplitude. The adjacent channel is thensensed, and, directly beside every good play-hack pulse in the channel,a pulse is recorded on the next channel. The latter channel then willhave pulses recorded onit which are directly in line with flawless areason the sprocket channel and the first referred-to channel. Thischannel-to-channel transfer continues until all information channelshave been inscribed. The synchronizing pulses originally recorded in thesprocket channel are then erased, and a final transfer is made from thelast information channel to the sprocket channel. The result of suchprocedure is to establish a sprocket channel con taining prerecordedsynchronizing pulses which are in line with only the good areas of allthe parallel information channels, and during subsequent operation alltape flaws are automatically dodged.

Use of a sprocket channel to fix the location of each information pulseor digit also makes it possible to selectively alter any amount ofinformation desired and thereby eliminate the necessity of any erasing.Suppose that a binary 1 has been stored at a given storage position, asdetermined by the sprocket channel. The magneticsurface of the tape atthis point is then saturated in a positive direction. Should it then bedesired to store a binary O in the place of the binary 1, it isnecessary then, in effect, to demaguetize the tape at that point and tomagnetically saturate that same area in the negative direction by simplysuperimposing a on the l. The sprocket pulse can be made to trigger arecording circuit at the precise instant that the small magnetized arearepresenting the l is underneath the air gap of the head, and therebycause the recording head to be pulsed properly to record a 0. If thiscurrent pulse is made of equal amplitude and duration but of oppositepolarity to the pulse by which the l was recorded, it will completelyreverse the polarity of magnetization at that point on the tape andleave it saturated in the negative direction. The binary 1 willtherefore be replaced by a binary 0, and in the same manner a 0 can bereplaced by a 1. Such described method of selective alteration obviatesthe need of preliminary erasure of a previously stored signal by virtueof the synchronizing control exercised by the sprocket channel.

The tape-drive mechanism The tape-drive mechanism employed with thisinvention consists of a known type of magnetic-tape-handling mechanismsuch as is manufactured by the Raytheon Manufacturing Company, Waltham,Massachusetts, Such mechanism is generally illustrated in Fig. l andwill be briefly described in order to make the disclosure complete.

The mechanism shown in Fig. l is designed to handle one-half inch wideplastic recording tape at a speed of .5 /2 inches per second in eitherdirection, and is furthermore capable of accelerating the tape from stopto full speed in either direction in a period of 5 to milliseconds. Asshown in Fig. l, the tape-drive capstan 101 is driven by aunidirectional constant-speed motor (not shown) through a system ofsolenoid-controlled dry magnetic clutches and a differential gearassembly. In order to make possible very rapid starts and stops, thecapstan, the gear assembly, and the magnetic clutches are designed tohave very low inertia and fast response. The immediate structure forproducing such desired characteristics does not form part of the presentinvention and is therefore not described in detail.

To the right of the capstan 1 31, there is mounted the transducer headassembly 162 consisting of six identical magnetic transducer headsmounted in a common housing. These heads are stacked side by side and inline across the width of the tape. The heads are precisely positioned inthe assembly so that the air gaps of all six are aligned to within a fewthousandths of an inch of a straight line perpendicular to the tapelength and so that the heads are evenly spaced across the width of thetape.

A pair of tape reels 103 are provided, which are mounted concentricallywith each other directly below the tape-drive capstan 101. These reelsare mounted on concentric shafts, and each is individually driven by aseparate motor. The motors are not shown but are mounted on the back ofthe panel which also houses a servo amplifier for controlling the reelholders. The tape 100 is threaded from one reel to the other in a closedloop arrangement, as shown in Fig. 1, over a series of fixed idlerpulleys 164 and a series of traveling idlers 10d mounted on a shiftablecarriage 1%, which in turn is mounted on a pair of guide rails 107. Theshiftable carriage 105 is connected by means of a link 108 to a variablereluctance pick-up 169. The pick-up 109 is in turn connected to a servoamplifier (not shown) mounted beneath the main panel shown in Fig. l.The specific construction of the magnetic-tape mechanism will not befurther described, since such tape mechanism is a part of the identifiedcommercially available tapehandling unit. However, the operation of thetape mechanism may be briefiy described since the immediate 6 inventionalso concerns the selective positioning of selected portions of the tapein order to obtain access to desired information.

In the idling position of the tape mechanism certain of the magneticclutch coils (14l71419, Fig. 14a) for the capstan 101 are energized in amanner to be described so that the capstan is held stationary, and theservo amplifier connected to the referred-to reel drive motors suppliesa sufficient and properly phased current to the motors so that they tendto rotate in opposite directions, their torques being balanced. The tapein this manner is held under constant tension, and it will be apparentby reference to Fig. 1 that the movable carriage 105 rests at the centerposition of its path of travel. To position the tape in a forwarddirection the proper clutch coils (Mid-4419, Fig. 14a) of the capstan101 are energized to cause forward rotation of the capstan. Theconstruction of the tape mechanism is such that the capstan comes up tospeed extremely rapidly and therefore exerts tension on one loop of thetape 10% due to the inertia of the tape reels; that is, the tape reelsdo not come up to speed immediately, and the reserve of tape which isstored between the reels and the capstan on the referred-to idlerscauses the carriage M5 to move in the direction of the greatest force.The movement of the carriage is reflected by a rotation of the link 198and the variable reluctance pick-up 109. The resulting change in signalfrom the reluctance pick-up N9 is fed into the referred-to servoamplifier which in turn controls the acceleration and torque of the tapereel motors. In other words, the only connection between the capstanfill and the tape reels 103 is through this electrical servo mechanism,the response of which is so fast that the movable carriage 105 shiftsonly approximately two inches from its center rest position before theservo mechanism will have sensed the acceleration of the tape by thecapstan and have brought the reels up to the proper speed. After suchcondition is reached, the tape will be moving forward at a specifiedspeed, the tape reels will rotate sufiiciently fast to supply and gatherup the tape at the proper rate for maintaining constant tape tension,and the movable carriage 165, after being displaced the said two inchesto the right of its center rest position, will remain in such position.

In a similar manner, to move the tape 100 in a reverse direction thesame sequence of events would occur with the exception that theshiftable carriage would be displaced approximately two inches to theleft of its center rest position.

The circuit employed to control the information stored and sensed fromthe magnetic tape 1% is shown in block diagram form in Fig. 2a. Theover-all operation of such circuit and its relation to the tapemechanism Will be briefly described, following which a detailedexposition of the various significant circuits employed will be given.

Recording circuitry (Fig. 2a)

As previously described, the head assembly 102 (Fig. 1) contains aplurality of adjacent magnetic heads or transducers, one for each of theinformation channels (including the sprocket channel) provided on thetape 109. The transducer associated with the sprocket channel will bereferred to as the sprocket channel head, and as shown in Fig. 2a theoutput signals obtained from such sprocket channel head are applied tosprocket amplifier 201. The function of the sprocket channel ampliher201 is to amplify the low level signals received from the sprocket headand to provide signals of sufficient amplitude to operate othercircuits. As described, all recordings in any of the informationchannels on the tape 100 must be done under the control of the sprocketsynchronizing channel. It is therefore necessary to sense thesynchronizing signals in the sprocket channel with the sprocket channelhead at virtually the same instant that another one of the transducerheads 102 is recording gamble 7 information in an adjacent channel whichis only a small distance away. Because of the small width of the tapeemployed, the windings of any two adjacent transducer heads aretherefore quite closely coupled when stacked in thehead assembly 82 and,despite the shielding between heads, considerable cross-talk ismanifested during a recording operation. In other words, the informationbeing recorded in a channel adjacent to the sprocket channel will bereflected as a spurious signal in the sprocket channel head anderroneous results would ensue. The design of the sprocket channelamplifier 231, together with the novel signal gating circuit, is such asto minimize error signals arising out of cross-talk.

Briefly, the synchronizing signals obtained from the sprocket channelare employed to control energization of the recording circuits but at aslightly delayed time interval with respect to the sensing of thesynchronizing pulses. Since the cross-talk signals induced in thesprocket channel head are generated by the recording signal employed inrecording information in an adjacent information channel, it followsthat the cross-talk signal must necessarily occur at a time intervalsubsequent to the sensing of the synchronizing pulse. Both the designedsynchronizing pulses and the unwanted cross-talk signals therefore occurperiodically but time displaced with respect to one another, and asuitable gating circuit is employed to block passage of the cross-talksignal to the recording circuit.

Sprocket-channel amplifier 201 (Fig. 3)

The sprocket-channel amplifier 201, which is detailed in Fig. 3, isspecially designed to cooperate with the transducers and the signalgating portions of the circuit shown in Fig. 2a to inhibit the effectsof cross-talk. For example, it has been determined that when a signal isapplied to a transducer which is directly adjacent to thesprocket-channel transducer, the resulting cross-talk measured at theterminals of the sprocket-channel transducer head is fifty or sixtytimes as large as the average reading signal normally obtained whenreading the magnetic tape. In actual operation, however, the recordingcircuit can be constructed so that these two signals do not occur atexactly the same time. That is, since a synchronizing signal obtainedfrom the sprocket channel of the tape is used to initiate the recordingof a signal in an information channel, the cross-talk signal therebyinduced in the sprocket-channel transducer head can be made to occur afew microseconds after the synchronizing signal from the synchronizingchannel has been detected. Advantage is therefore taken in the presentinvention of such time difference to insure that reading of the sprocketchannel signal will occur only during the interval when no cross-talksignal is present and the subsequent occurrence of any cross-talk signalis employed as a gating signal to prevent reading of information by thesprocket-channel head when cross-tall; is present. In other words, thesprocket-channel amplifier 291 can be designed to ignore the relativelylow level synchronizing signals in the information channel during theinterval when any crosstalk signal is present and in the absence of anycross-talk signal the amplifier is designed to provide sutlicientamplification of the normal low-level information signal, which has beenrecorded in the sprocket channel, and to recover from any of thereferred-to crosstalk signals in time for the next synchronizing signal.A complete circuit for the sprocket-channel amplifier 2'61 is shown inFig. 3. At various points in Fig. 3 there are indicated the pertinentfigures in the drawings which illustrate the wave forms of the signalsexisting at such points in the amplifier.

The average amplitude of the signal obtained by reading thesynchronizing signal pulses stored in the sprocket channel isapproximately 1 millivolt. The undesired crosstalk signal appearing atthe input of the amplifier 201 usually amounts to approximately 50millivolts. Inorder 8 to amplify the desired 1 millivolt in ormationsignal, the first stage of the amplifier 201, detailed in Fig. 3, isdesigned to have a linear'gain of 25. The first stage of thesprocket-channel amplifier comprises the tube V301a having a gridreturned to ground through a relatively small resistor 3&2. The cathode303 is directly connected to ground. Since amplifier tube V301a is notself-biased, the occurrence of a large SO-millivolt cross-talk signalwill cause grid current to fiow during the large positive sweep of suchsignal, and characteristically, a certain amount of limiting willtherefore occur. In such manner, the sprocket amplifier 201 functions toreduce the amplitude of the spurious signals arising out of cross-talkto a point where such signals will not cause blocking of the amplifier.Referring briefly to Figs. 9a-9c, a typical sig-, nal 10!), representingcross-talk, is shown in Fig. 9b and is comparable in amplitude to thedesired intelligence signal 10 after amplification. The functioning ofthe amplifier in obtaining such result can be explained by reference tothe oscillograms showing the signals at various points in the amplifier.

Figs. Sa-Sc represent oscillograms of the actual output signals obtainedfrom the first stage V301a, of amplifier 261 shown in Fig. 3. Fig. 5arepresents the normal low-level synchronizing pulse signals as read fromthe sprocket channel on the tape. Fig. 5b illustrates the magnitude andshape of the amplified cross-talk induced in the sprocket-channeltransducer head when binary ls are recorded in a channel adjacent to theprocket channel. Fig. 50 illustrates the output signal obtained from thesprocket-channel amplifier tube V301a as a result of cross-talk inducedin the sprocket channel head when binary Os are recorded in a channeladjacent to the sprocket channel. in other words, during normaloperation, there may be applied at the input of a second stage V3tl1b,of amplifier 201, all three of the signals illustrated in Fig. 5, and itis desired to obtain a response only in respect to the desiredsynchronizing signals (Fig. 5a). The relative amplitudes of the readingand crosstalk signals are apparent by comparing Figs. 5b or 50 with Fig.5a. The second stage V3011; of the amplifier is of conventional design,but the third stage V302a includes a pair of oppositely connected diodes304, 305 in the grid circuit, and a cathode resistor 303. Diode 304 willconduct upon the occurrence of an applied signal which is below ground,while diode 305 will conduct upon the application of a signal aboveground. The effect of the diodes is to make the maximum effectivevoltage gain of stage V302a less than two in the case of the readingsignal for example, and to further cause the stage gain to decreasenonlinearly as the amplitude of the applied signal increases. Suchaction will be apparent from a consideration of the typicalcharacteristic curves for a germanium crystal type diode illustrated inFig. 6.

In accordance with Fig. 6, as the forward voltage across a diodeincreases, the forward resistance of the diode will decrease in anonlinear fashion. The diodes 304 and 385, shown in Fig. 3, thereforemake the effective load resistance across amplifier tube V3011; anonlinear function of the signal amplitude in such a manner that theload resistance decreases as the signal amplitude increases. Since, fromFig. 5, the undesired cross-talk signals (Figs. 5b, 5c) at this stage inthe amplifier are so much greater than the desired synchronizing signal(Fig. 5a) such characteristics of the load resistance can beadvantageously employed to minimize the effect of the cross-talk signal.At the output of the amplifier stage Vittllb then. the ratio of thecross-talk signal to the reading signal amplitudes will be reducedapproximately 5 to l, and furthermore, no noticeable distortion of thereading signal wave shape will occur. The attainment of an effectivereduction in the relative amplitude of the cross talk signals isparamount to the sacrifice in the amount of amplificationof the desiredsynchronizing signal con sequent to such design.

The fourth stage of amplification V302b, is also characterized by anonlinear circuit in the grid comprising the diodes 305, 307. Diodes306, 307 function in a manner similar to that described in connectionwith stage V302a, so that the gain characteristic of this stage issimilar to that or" the preceding stage. The effect is such that theoutput of stage V 302b, the normal gain of which would be 50 or 60,yields a cross-talk signal which is now reduced to less than twice theamplitude of the desired synchronizing signal and, as a result, thesensed synchronizing signal instead of the cross-talk signal dominatesthe control action of the amplifier. Fig. 7 shows the wave form of thesignals as they exist at the output of V30b.

As contrasted with the wave forms show in Fig. the desired synchronizingsignals read from the sprocket channel on the magnetic tape now have thecharacteristics illustrated in Fig. 7a, while the cross-talk signalsobtained from the output of V3021: are illustrated in Figs. 7b and 7c,corresponding respectively to cross-talk signals occasioned by therecording of binary 1s and binary 0s in an adjacent channel. As a resultof such comparison, it is apparent that the effect of the stages ofamplifier 201 so far described is to enhance the desired synchronizingsignals (Fig. 7a) as compared to the suppressed cross-talk signals(Figs. 7b, 70) so that the magnitude of the cross-talk signal is lessthan twice the amplitude of the desired intelligence signal. This is incontrast to the referred-to 50-to-1 ratio originally present at theinput to the amplifier. Referring again to Fig. 3, the differentiatingcircuit comprising the capacitor 309 in the output circuit of amplifierV302b and the resistor 310 in the grid-return circuit of amplifier tubeV311 serves primarily to ditferentiate the signals read from thesprocket channel and to supply essentially unidirectional pulses to thelast stage V311 of the amplifier 201. However, the presence of suchcircuit also serves to differentiate the cross-talk signals (Figs. 7band 70) which are also present at this stage.

The differentiated signals present at the grid of amplifier stage V311are illustrated in Figs. 8a-8c, while Figs. 9a9c show oscillogramsobtained from the final stage V311 of the sprocket-channel amplifier.The three oscillograms shown in each of such figures correspondrespectively to the desired synchronizing signal and the crosstalksignals induced in the sprocket-channel head and the recording of binary1s and Os, respectively, in an adjacent channel.

The final amplifier stage V311 consists of a pentode wh ch is operatedat zero bias due to the absence of a bias resistor in the cathodecircuit. Such stage can therefore amplify only the negative-goingportions of the signals shown in Figs. 8a8c applied at the input grid ofV311. The signals obtained from the output of V311 are shown in Figs.9a-9c. Since the output obtained from stage V311 includes the cross-talksignals, such as the wave form 1% shown in Fig. 9b, the use of afeedback circuit including a delay multivibrator can now be employed toobtain the referred-to desired action and thereby eliminate or inhibitthe cross-talk signal.

In order to achieve such gating action, the referred-to output signals,such as in Fig. 9b, are applied to a Schmitttype trigger circuit 2-02,which provides a rectangular pulse having a fast rise and fall time foreach synchronizing signal received from amplifier 201. Figs. l0-10c showthe character of the output signals obtained from the Schmitt circuit.That is, the signals represented by Figs. 10a, 10b, and 100 representrespectively the output signals (Figs. 9a, 9b, 9c). The Schmitt triggercircuit is of known construction and is described in detail on pages99-103 of Electronics Experimental Techniques, by Elmore and Sands,published by McGraw-Hill (1949).

The characteristics of such circuit, as clearly described in thereferred-to text citation is to deliver an output in response to anapplied input signal only when the input signal teaches a predeterminedlevel. The Schmitt circuit is adjusted so that should a wave form of thetype illustrated in Fig. 9a: be applied, such circuit will be triggeredonly when the applied signal reaches a level corresponding toapproximately 30 percent of its full amplitude for an average signalfrom the sprocket amplifier. Such selected trigger level is safely abovethe output noise level of the amplifier and yet allows for variations inthe amplitude of the signals obtained from the sprocket amplifier. Thesquare wave output thereby obtained from the Schmitt circuit representssignificant signals which are independent of spurious signals arisingout of background noise.

The Schmitt circuit 202 also includes a difierentiating circuit and Fig.11 shows the same pulses discussed in connection with Fig. 10 after suchdifferentiating occurs. The sharp differentiated pulses are employed astriggers which are applied to the control grid of a gating tube 203(Fig. 2) and the output of gate 203 is applied through pulse shaper 204to delay multivibrator 205 through lead 204a. When triggered, the pulseshaper 204 provides a positive pulse of approximately 2 microsecondsduration. An output line 20% is provided to feed such pulses to theover-all system with which the storage device is used if desired. Thesepulses are normally employed to synchronize the transfer of informationinto and out of the tape.

Referring again to Fig. 2a, a delay multivibrator 205 is inserted in afeedback connection 204a, 205a, between the output of pulse shaper 204and a second input of gate 203. The delay multivibrator 205 may be aconventional cathode-coupled monostable multivibrator adapted togenerate timing wave forms in a known manner as described and shown onpage 181 of Waveforms, vol. 19, of the Radiation Laboratory Seriespublished by McGraw-Hill. Conduction of such multivibrator is initiatedby a pulse rorn pulse shaper 204 and applied to the delay multivibratorthrough conductor 20 5-0. The negative output of the multivibrator ischosen. Since the output signal is a square wave having a fixedamplitude of definite duration, it can be used to block the conductionof gate 203 for a time interval corresponding to the duration of suchpulse, as will be apparent.

The gate 203 is a conventional 6AS6 type of multigrid tube used as agating device. The circuit for gate 203 is detailed in Fig. 178. Theoutput of the Schmitt circuit 202 is applied through lead 202a to thecontrol grid of the gate tube, and the output of delay multivibrator 205is applied to the suppressor grid through conductor 205a. The gate 203includes a pulse transformer in the plate circuit as shown in Fig. 17e,so that an output of desired polarity may be obtained from thetransformer secondary.

The suppressor grid of the gate tube is normally held at groundpotential, and the application of a positive signal or pulse to thecontrol grid will therefore initiate conduction. However, theapplication of the described negative output square wave pulse fromdelay multivibrator 205 to the suppressor grid will act to blockconduction of the gate for the duration of the square wave pulse. Thatis, because of the square wave form, the gate is turned off instantlyand held off during the microsecond duration of the square timing wave.The purpose of such arrangement will now be described.

At this point in the description it will be clear that signals havingthe characteristics of either Fig. llb or 11c, depending on whether abinary 1 or 0 is being recorded in adjacent information channel, will beapplied to the control grid of gate tube 203. Wave form corresponds tothe desired synchronizing signal, while wave form 111 represents thespurious signal arising out of cross talk.

Since the gate 203 (Fig. 2) is turned off a fraction of a microsecondafter it has been triggered to conduct-ion 11 i by a signal such as thereferred-to pulse 110, and since the gate is further held nonconductiveby the referred-to 100 microsecond output pulse from delay multivibrator205, it is apparent that the unwanted cross-talk signal pulse 111, whichoccurs a few microseconds subsequent to pulse 110, as will be shown,cannot get through the gate. After the referred-to 100 microsecondinterval has elapsed, the gate is again opened to receive the nextdesired synchronizing pulse from the sprocket channel.

Thus, by providing the described IOO-microsecond duration delay timingpulse, which corresponds approximately to the IOU-microsecond intervalbetween the prerecorded synchronizing pulses in the sprocket channel forblocking the gating tube between each desired synchronizing pulse, it iseasily possible to entirely eliminate the deleterious effects occasionedby the cross-talk signals.

In such described manner there is present at point 206 in the blockdiagram of Fig. 2a only the desired synchronizing signals obtained byreading the sprocket channel on the tape, the cross-talk signal havingbeen eliminated as a possible triggering source in the described manner.The synchronizing signals are thereafter employed in the remainingportion of the block diagram of Fig. 2a to regulate the recording andreading of information in the information-storing channels on tape 100.

In order to facilitate the separation of the spurious signals arisingout of cross-talk due to a recording operation from the desiredsynchronizing pulses, each synchronizing pulse is given a slight delaybefore it is applied as a control pulse in the information-recordingcircuit. By virtue of such delay, which may be of approximately 15microseconds duration, the recording signal and the cross-talk signalconsequent thereto is restricted to occur 15 microseconds after asynchronizing pulse has been sensed. This permits the described methodof discrimination between the synchronizing and cross-talk pulse to beeffective. To provide such a delay, a second delay multivibrator 207,similar in construction to the multivibrator 205, previously identified,is employed. In this instance, however, the multivibrator is used in aslightly different manner, as shown and described in Radio Engineering,by F. E. Terman (third edition) on pages 590591. The application of asharp triggering pulse, such as is represented by the synchronizingpulse obtained from the output of pulse shaper 204 to the input ofmultivibrator 207 results in a square wave output, the duration of Whichis readily determined by the circuit constants as is well known. It willbe appreciated that the time interval between the leading and trailingedges of the square wave output can be used as a delay function. Sucheffect is accomplished by differentiating the square wave output througha suitable resistor-capacitor combination to produce sharp positive andnegative pulse corresponding respectively to the leading and trailingedges of the square wave as indicated in Fig. 2a. The trigger amplifier208 is biased to select and amplify only the pulse derived from thetrailing edge of the square wave, which pulse is applied through asecond pulse shaper 209 to point 210 in Fig. 2a. Such pulse represents a15-microsecond delayed sprocket pulse at point 210, as compared to thesprocket amplifier pulse manifested at point 206. These delayed sprocketpulses perform the function of timing each pulse recorded in aninformation channel on the tape and are therefore not employed during areading operation.

The synchronizing pulses obtained at point 210 in Fig. 2a are employedto control the recording of information in the channels adjacent to thesprocket channel. The input terminals 211, 212, represent the points ofapplication of the information or instructions to be stored as obtainedfrom the particular over-all device with which the recording mechanismis used.

The manner in which the synchronizing pulses from the sprocket channelcooperate with the applied information signals to control the recordingof information through the transducer heads 102 is based on an arrange-Quicken 12 r ment of gating circuits whereby coincidence between thedelayed synchronizing signals and either of the information signalspresent at input points 211, 212, will result in the recording of theproper information. Complete control of all recording is achieved byproperly gating the print and print ls signals applied at terminals 212,211, with the delayed synchronizing pulses from the sprocket channel.

The portion of the circuit shown in Fig. 2a from point 210 to the recordtransformer 220 is involved in recording the two classes of informationwith which this invention is concerned.

The upper branch comprising the elements 213, 214, 216, and 218determines the recording of binary ls, while the lower branch 215, 217,219 controls the recording of binary Os.

The terminals 211 and 212 correspond respectively to the point ofapplication of a print 1 and print signal, respectively. Suchinformation signals are obtained from a source, not shown, comprising anelement of the overall system with which the present device may beemployed; for example, the National Bureau of Standards EasternAutomatic Computer (SEAC), significant portions of which are describedin an article entitled SEAC," by Greenwald et al., Pros. I. R. E., vol.41, October 1953, pp. 1300-1313, and which is in public use. When it isdesired to record an output information signal with such type of device,a positive direct-current print signal will be supplied to terminal 212from the over-all device. In addition, if the information to be storedis a binary 1, another significant positive signal will be applied atinput terminal 211. A binary 0 will be manifested by the absence of anysuch significant signal.

If a print or storing instruction signal appears at terminal 212, itwill be simultaneously applied to both and-gate 214 and the and-inhibitgate 215. These gates are of known construction and are shown in Figs.17a, 17b and 17c, 17d, respectively. As is well known, an and-gate(Figs. 17a, 17b) will deliver an output signal only upon concurrence ofa plurality of applied input signals and in the absence of a negativeinhibit signal. The presence of a negative inhibit signal (Figs. 17c,blocks the gate. The construction and operation of such logical circuitsis fully described in an article entitled, Dynamic circuit techniquesused in SEAC and DYSEAC, by Elbourn and Witt, Proceedings of the I. R.13., volume 40, No. 10, October 1953, pages 1380-1383.

Assuming then that a printing instruction signal has been delivered toterminal 212, the subsequent application of either print ls or theabsence of such signal at input terminal 211 will ultimately determineenergization of either the upper branch (213, 214, etc.) or the lowerbranch (215, 217, etc.) in Fig. 2a.

Specifically, the application of a print-ls information signal toterminal 211 concurrently with the arrival of a delayed synchronizingpulse from pulse shaper 209 will cause and-gate 213 to produce both apositive and negative output signal on leads 213a and 213b,respectively. The application of the negative output pulse through lead213!) will immediately inhibit or cut off and-inhibit gate 215 whichthereby prevents subsequent energization of the lower branch portion ofthe circuit.

In the absence of a print ls information signal, no output will beobtained from and-gate 213, and hence gate 215 will not be inhibited.Therefore the application of a synchronizing pulse from point 210 intime with a print instruction signal from 212 will produce an outputfrom and-inhibit gate 215 and cause energization of the one-shotmultivibrator 217, the Os record amplifier 219 and record transformers220, which will inscribe a spot of negative polarity on the tape 100.

Returning to the condition in which a print-ls signal is applied atterminal 211, upon concurrence of a synchronizing signal at timedintervals from pulse shaper 209, with a print-ls signal there will beproduced positive and negative outputs from and-gate 213. Thenegcomponent 202 shown in Fig. 20.

ative output obtained from gate 213 will cut off and-inhibit gate 215,while the positive output will be applied through lead 213a to a secondand-gate 214. Concurrence of such positive output signal with thereferred-to print instruction signal obtained from terminal 212 willenergize gate 214 to produce an output which is applied through lead214a to the input of one-shot multivibrator 216. Vibrators 216 and 217are of identical construction and are of a known type such as isdescribed on pages 590-591 of Termans Radio Engineering, third edition.In response to such triggering, multivibrator 216 will produce a squarewave output pulse of 10 microseconds duration which energizes thels-record driver amplifier 218 and record-transformer 220 to produce apositive magnetizing spot on the tape.

It is apparent therefore that the time-spaced synchronizing pulsesobtained from the sprocket-channel amplifier 201 determine theenergization of both the 1s and Os recording channel so that recordingof either type of information can occur only in synchronization withsuch timing pulses. Selection of either a ls or Os recording channel isdetermined by the nature of the information signal applied at terminal211.

Summary of operation (recording circuitry) Assuming that thesynchronizing pulses have been prerecorded in the sprocket channel atprecise 100-microsecond intervals, then the sprocket-channel amplifier201 will provide such synchronizing pulses at point 206 in Fig. 2a. Thedelay circuit 207 then provides a IS-microsecond delay to each suchsignal before it is applied to point 210. Since such delayedsynchronizing pulse determines energization of either the ls recordingcircuit or the Os recording circuit by controlling the gates 213, 215,all recording of information must be initiated at a time intervalsubsequent to that in which a synchronizing pulse has been sensed. It isapparent therefore that any crosstalk signal induced in thesprocket-channel head arising out of a recording operation in anadjacent information channel must correspondingly occur at al5-microsecond interval subsequent to the desired synchronizing signalas described in connection with Figs. 9-11. Since the desiredsynchronizing pulses and the unwanted cross-talk signals each occur at100-microsecond periodic intervals, but time-displaced with one another,the gating tube 203 together with the delay multivibrator, function toblock passage of the unwanted cross-talk signal by properlysynchronizing the gating action of tube 203 with the synchronizingpulses in the described manner.

Reading circuitry (Fig. 2b)

The amount of circuitry involved in a reading operation is considerablyless than that required for recording, since most of the critical timingis necessary only during recording. Reading consists simply of detectingthe signals stored on an information channel, determining Whether eachinformation signal represents a 1 or a 0, and transferring suchinformation to a desired utilization device as a sequence of synchronouspulses. Fig. 2b is a block diagram of the circuitry which performs thesefunctions.

The information channel amplifier 221, to which the transducer heads 102for the information storage channels are connected, is a conventionalclass A amplifier consisting of four triode stages and having anover-all .voltage gain of 100,000. The output signals from the amplifierare approximately 100 volts in amplitude and drive a Schmitt circuit 222which is triggered only by the positive half cycle of each signal fromthe amplifier and provides a BO-microsecond rectangular pulse output.The construction of the Schmitt circuit 222 is similar to that describedin connection with the description of The output pulse passes through animpedance-matching cathode follower 223, and is applied to one input ofan and-gate 224 which is the same type as identified in Fig. 17a. Theundelayed pulses from the sprocket. channel as obtained from pulseshaper 204 are applied to the other input of the and-gate 224. During areading operation these pulses are derived from the sprocket channelexactly as they are during recording, and even though there is nocross-talk signal to disturb the circuits during reading, the timing ofa given synchronizing pulse from the sprocket channel is the same forboth operations.

Fig. 12 is a timing chart showing the relationship among the signalsexisting at various points in the circuit under consideration, asfollows:

Fig. 12a shows the synchronizing pulses as they exist in the sprocketchannel;

Fig. 12b shows the delayed synchronizing pulses as they exist at point210 in Fig. 2a;

Fig. illustrates the recording current employed for recording a binary 1or a binary 0 as indicated by Fig. 12a;

Fig. 12d illustrates the flux distribution on an information channel dueto the recording currents shown' in Fig. 12c;

Fig. 12e shows the playback wave form obtained as a result of readingeither a binary 1 or a binary 0 from an information channel the natureof the digit being told in Fig. 12i;

Fig. 12 indicates the output obtained from the reading channel Schmittcircuit 222 corresponding to the wave form of Fig. 126;

Fig. 12g is a repetition of the synchronizing pulses shown in Fig. 12ashowing the relation of such pulses with the Schmitt circuit outputduring a reading operation;

Fig. 12h shows the output obtained from gate 224 during a readingoperation.

Fig. 12i shows the nature of the binary digits related to the above waveforms.

The and-gate 224 combines each information signal read from aninformation channel (Fig. 122) with its associated synchronizing pulseFig. 12a thereby determining whether the sensed information signalrepresents a 1 or a O, as will be clear by reference to the timingdiagram in Fig. 12. From Fig. l2e it can be seen that the playbacksignal obtained by sensing a binary 1 has an initial positive swingwhich is followed by a negative swing whereas a signal having oppositecharacteristics is obtained in reading a 0 (compare the first and thirdwave forms in Fig. 12e). The value of an information digit is easilydetermined, therefore, merely by sensing whether the first half cycle ofthe playback signal (Fig. 12c) is positive or negative. Accordingly, theplayback signals (Fig. 12e) from the information channel amplifier 221,when applied to Schmitt circuit 222, will be discriminated in a mannerso as to provide only a positive output during each positive swing asshown in Fig. 12 These signals from the Schmitt circuit are thencompared with the synchronizing pulses 12a or 12g by the and-gate 224.If the sensed digit is a l, the Schmitt circuit signal and the sprocketpulse will coincide as indicated by broken line a in Fig. 12, resultingin a pulse output from gate 224. If the information digit is a 0, thesprocket pulse and the Schmitt signal will arrive at the gate atdifferent times, and there is no coincidence as shown by lines bb inFig. 12, and hence no output is obtained from gate 224. During a readingoperation, a pulse output from the gate at a given sprocket pulse timetherefore indicates a 1, and no pulse from the gate at a given sprocketpulse time indicates a 0.

At this point it should be clear why it is necessary during recording todelay the sprocket pulses in element 207, Fig. 2a, before they triggerthe record circuits. If the sprocket pulses Were not delayed duringrecording, then timewise, the magnetic flux (Fig. 12d) representing theinformation digit would be centered with respect to the sprocket pulse(Fig 12a) and, during reading, the sprocket pulse would not thereforecoincide with the first half cycle of the information signal but wouldoccur at the time when the information signal is crossing the base line.Such operation would be marginal, of course, and very unreliable.Delaying the sprocket pulses which trigger the record circuits overcomessuch difficulty. The microsecond delay during recording shifts theinformation digits just enough with respect to the sprocket pulses sothat on playback a sprocket pulse always occurs during the first halfcycle of each information signal, and reliable coincidence is assured'Miscellaneous associated circuitry In addition to the main circuitryinvolved in the recording and reading of information other circuitry isnecessary to perform such associated functions as switching informationchannel heads and controlling the tape transport mechanism. In systemswhere the same head is used for both the recording and reading ofinformation, the usual procedure is to switch the connections to thehead so that during reading the head is connected only to the input ofthe reading amplifier and during recording it is connected only to therecording circuits. The purpose of such switching is to isolate thereading amplifier input from the recording circuits so that there is nointeraction between the two. In order to eliminate the necessity of suchseparate switching operation, the circuit shown in Fig. 13 is employed.This circuit makes it possible to leave the recording circuits and thereading amplifier permanently connected to each other, so that it isnecessary only to connect the transducer head of the selected channel toa pair of common terminals to read or record.

In Fig. 13 the ls and Os record amplifiers 213 and 21?, identified inconnection with the description of Fig. 2a are shown in circuit with thepreviously referred-to recordtransformer 220. The transformer secondaryis connected to the reading amplifier 221 previously identified inconnection with Fig. 2b through a circuit including a plurality ofgermanium type diodes 1301-1304. The transducer heads for each of theplurality of channels on the tape are designated as A, B, C, D, and E,respectively, in Fig. 13, and the energizing coil for each head ispermanently connected to respective pairs of contacts, A A B B etc., asshown.

A selector relay including a solenoid such as A and a double armaturesuch as A is provided for each of the heads, the armatures being adaptedto complete a circuit between the contact pairs A A etc., and thereferred-to circuit between the record and read amplifiers 218, 219, and221, as shown in Fig. 13. To select a given information channel foroperation a positive D.-C. voltage is applied to the terminal anode as Aof a channel-selectorrelay, and the armature (such as A will thencomplete a circuit between the selector channel ahead and the readingand recording circuit as shown in Fig. 13.

The four germanium diodes 1301-1304, shown in the circuit of Fig. 13,permit automatic coupling between the transducer heads and either therecord or read amplifier. The magnitude of the two signals involved inrecording and reading differ greatly, and the nonlinearity of the diodesis used to advantage. By using the circuit shown in Fig. 13 it has beenempirically determined that for a l0-microsecond -milliampere recordingpulse through the head the total equivalent resistance of the two diodes1301, 1302., in series with the head is 130 ohms, the inductivereactance of the head is approximately 3,300 ohms, and the equivalentresistance of the two grid-return-diodes is 120 ohms. Thus, the head isshunted by approximately 4,000 ohms and has 130 ohms in series with itand the current source during a recording operation. The voltage dropacross the series diodes is negligible, but approximately 45 percent ofthe pulse current available from the transformer 220 will pass throughthe 3.9K resistor 1305 and the diodes 1303, 1304, in shunt with thehead. While this current is wasted, the

(forward) and R (reverse drive).

tubes which drive the transformer are capable of supplying three to fourtimes the pulse current needed for recording and the loss is easilytolerated. When current flows through the shunt path, the 3.9K resistor1305 and the two grid-return diodes 1303, 1304, act as a voltagedivider, and since the diodes have an equivalent resistance of only 120ohms, most of the drop in potential appears across the resistor.Actually only three percent of the recording voltage applied to the headappears at the grid of the reading amplifier tube, and this is smallenough to be tolerated by the reading amplifier.

During a reading operation, the playback signals obtained from the headare approximately one millivolt in magnitude and consequently all fourdiodes 1301-1304 in the circuit have a much higher equivalentresistance. The resistance of the series diodes 1301, 1302, plus thehigh impedance of the secondary of transformer 220, is extremely highduring reading, so that the voltage divides across resistor 1305 and thegrid-return diodes 1303, 1304. It is obvious that the signal applied tothe grid of the tube would be too small unless the equivalent resistanceof the diodes at least approaches 3.9K. Actually, this equivalentresistance is approximately 6.7K so that the voltage division is in thedesired direction and at least two-thirds of the playback signal isapplied to the grid of the reading amplifier 221. Therefore the circuitsuccessfully provides electronic switching for both reading andrecording and mechanical switching of heads is necessary only when adifferent tape channel is selected.

Control of tape drive The described means for electronically controllingthe storing of information on the tape requires that the tape betransported and positioned with a precision commensurate with thatprovided by the signal control means.

The commercial tape mechanism described in connection with Fig. 1includes an internal control circuit designed to be normally controlledby three timed trigger pulses provided by an external sourcecorresponding respectively, to tape forward, tape reverse, and tapestop." The original control circuit included amplifiers which controlledthe firing of a plurality of thyratrons connected to the clutch coils ofthe capstan described in connection with Fig. l.

The present invention includes a necessary modification to such controlcircuitry which permits operation of the tape mechanism under thecontrol of only two instead of three control signals as provided for inthe instrument as supplied. The purpose of such modification is to adaptthe tape mechanism for use with an over-all system of the type describedin the referred-to I. R. E. publication which provides only two controlsignals for the magnetic tape storage system: Tape forward and tapereverse but no tape-stop signal.) The modified circuit is completelydetailed in Fig. 14. Such circuit is designed to exercise all threeconditions of control over the tape drive in response to only twoapplied control signals.

Referring to Fig. 14 the connections for applying the referred-to tapeforward and tape reverse signals as obtained from the overall mechanismare identified as terminals 1406 and 1407. The ganged manually settableswitches have a plurality of selection contacts corresponding to C(outside control), S (stop position), F Such symbolization isself-explanatory. With switch 1408 positioned at position C, the tapemechanism will be automatically controlled by signals applied toterminals 1406, 1407, from the over-all mechanism with which the tapemechanism is used. Contacts S, F, or R are selected in the event it isdesired to manually control the tape mechanism for stopping, forwardadvance, or reverse movement, respectively.

The coils designated 1417, 1418, 1419, and 1420 in Fig. 14a representthe referred-to clutch control coils for determining the movement of thecapstan drive 101.

17 Energization of coils 1417 and 1419 holds the capstan stopped;energization of coils 1418 and 1419 produces forward rotation of thecapstan drive, and when coils 1420 and 1417 are energized the capstanwill run in a reverse direction. Such information is tabulated on thechart of Fig. 14b for reference purposes.

Tape stopped Selective energization of the coils 1417-1420 in therequired combinations is determined by the firing of the thyratronsV1413, V1414, V1415, and V1416, associated with each of the coils. Thenormal positions of the armatures associated with each of the relays1411 and 1412 are shown in Fig. 14a as completing a circuit acrosscontacts 1411a and 1412a, respectively, thereby applying a positivepotential of 62 volts to the control grids of the thyratrons V1413 andV1415. of coils 1417, 1419, corresponds to a stopped condition of thecapstan drive as shown in the chart of Fig. 14b. The relays 1411 and1412 are in the plate circuits of tubes V1409, V1410.

With the switches 1408a and 1408b set as shown for automatic control (C)of the tape mechanism drive, a tape forward signal will be automaticallyapplied to the grid of V1409 and a tape reverse signal to the grid ofV1410, should such control signals be present at terminals 1406 and1407. Because of the illustrated diode arrangement in the grid circuits,these tubes are normally biased to cut-off. The second grids of each ofthe tubes V1409, V1410, are connected respectively to an armature of apair of relays 1404, 1405, each forming the plate load of amplifiertubes V1403a and V1403b. The normal position of the armatures for relays1404, 1405, are as shown in Fig. 14a and a positive potential of 62volts is therefore normally present in the screen grid of tubes V1409and V1410.

The presence of a positive tape forward signal at terminal 1406 willtherefore cause V1409 to conduct and throw the armature of relay 1411 toa position opening the circuit across contacts 1411a and applying apositive voltage through contact 141112 to the control grid of tubeV1414. Such action will cause V1414 to conduct and the resulting initialnegative pulse will be transmitted through coupling capacitor C1421 tocut-off tube V1413. Since thyratron V1415 has remained in a conductivestate, relay 1412 not having been energized as a result of the aboveaction, clutch coils 1418 and 1419 are now in an energized state, acombination which, according to the chart in Fig. 14b, results in aforward drive of the capstan.

Tape reverse drive Similarly should a tape reverse signal be present atterminal 1407, the action of the armature of relay 1412 and contact1412b will be such as to fire thyratron V1416, cut-off thyratron V1415,and energize coil 1420. Under such conditions no tape forward signalwill be present at terminal 1406, and hence the armature of relay 1411will remain in the position shown in Fig. 14a and coil 1417 will remainenergized. As indicated by the chart of Fig. 14b, energization of bothcoils 1420 and 1417 indicates a reverse drive of the capstan.

In the described manner the tape drive is completely controlled by onlythe two tape forward and tape reverse signals obtained from the over-allapparatus as long as switch 1408 is in the C position. In the otherpositions the switch 1408 controls the same circuit elements in a likemanner to manually control movement of the tape.

The remaining circuitry shown in Fig. 14a, including the tubes V1402,V1403, and relays 1404 and 1405, provides automatic end stopping of thetape to prevent inadvertent separation of the tape from either reel dueto overrunning in either direction.

The resulting energization 18 End-stop senser An auxiliary sensingdevice 1500 is mounted on the tape mechanism shown in Fig. 1 adjacent tothe concentric reels and in contact with the non-magnetic side of thetape. The construction of the end-stop senser is detailed in Fig. 15 andcomprises a base plate 1505, made of insulating material on which aplurality of conductive contacts 1501, 1502, and 1503 are mounted. Thecontacts 1502 and 1503 are insulated one from the other by an insulatingcollar 1504. As shown in Fig. 16, a mark 1601 made with conductivesilver paint is applied on the nonmagnetic face of tape acrossapproximately one half of the tape width and a corresponding conductivemark is painted on the same face of the tape at a point adjacent to theother end of the tape and occupying the opposite width of the tape, asis clearly indicated in Fig. 16.

The sensing head 1500 is mounted so that the tape 100 runs in physicalcontact with all three of the conductive contacts 15011503, and thewidth of the head corresponds to the tape Width. The physicalarrangement of the conductive marks 1601, 1602, and the spacing of thethree contacts 1501-1503, permits each of the conductive strips 1601 and1602, respectively, to bridge separate pairs of the contacts 1501-1503.That is, the disposition of strip 1601 with relation to the width of thetape is such as to bridge contacts 1501 and 1502, while strip 1602 canbridge contact 1501 and contact 1503. In other words, the respectivepairs of contacts which have been bridged in such manner determine thepresence of either end of the tape.

The contacts 1502 and 1503 are also shown in Fig. 14a as being connectedto the input of amplifiers V1402a and V1402b, respectively. The commoncontact 1501 is connected to a negative voltage source as shown. The'ends of the tape 100 are also symbolically illustrated in Fig. 14a as isalso the bridged position of the conductive strips 1601, 1602. the tape100 has been reached, the strip 1601 will bridge contacts 15011502 andcontact 1502 will deliver a negative pulse to the grid of V1402a.Similarly, when the other end of the tape is reached, conductive strip1602 will bridge contacts 1501 and 1503 to apply a negative signal tothe grid of V1402b.

Should the sensing mark 1601 operate contact 1502, as is the case wherethe tape is traveling in a forward direction (i. e., coils 1418, 1419energized, relay 1411 energized and relay 1412 deenergized) a positivepulse will cause driver tube V1403a to conduct and energize relay 1404which is in the plate circuit of V1403a. The double armature of relay1404 is shown in its normal position in Fig. 14a, and when thrown by theresulting energization of relay 1404 one armature will close a holdcircuit established by contact 140401. Contact 1404b serves to groundthe screen grid of tube V1409, which is otherwise normally at a positivepotential and tube V1409 is thereby cut off even though a positive tapeforward pulse should then be present on its control grid.

Since relay 1411 is thereby deenergized, its armature will resume thenormal position shown in Fig. 14a, which results on conduction of tubeV1413 and energization of capstan coil 1417. Since, according to thechart of Fig. 1411, during the forward motion of the tape, coils 1418and 1419 were energized, the referred-to positioning of the armature1411 will now have cut off V1414, deenergizing coil 1418. Therefore,since coils 1417 and 1419 are now energized the capstan drive and thetape will be stopped, as is evident from Fig. 14b.

Forward movement of the tape is therefore automatically halted when theforward end of the tape has been reached.

The end-stop senser functions in a similar manner to stop the tape drivewhen the tape is moving in the reverse direction and approaches the endof the reel. When the tape is running in a reverse direction, clutchcoils 1420,

It is evident that when one end of 1417, are energized, as shown by thechart of Fig. 14b. In such condition relay 1411 will be in its normaldeenergized position and relay 1412 will be energized. When the tapeapproaches the end of the reel, mark 1602 (Fig; 14a) will bridgecontacts 1501 and 1503 to initiate a positive pulse in relay 1405. Thedouble armature of such relay will be thrown from the position shown inFig. 14a to a position contacting relay contacts 1405b, 1405a.

Tube V1410 will therefore be cut off and the consequent deenergizationof the relay 1412 in the plate circuit will shift the armature relayback to the normal position shown in Fig. 1411. Tube V1416 will therebybe cut off and coil 1420 deenergized, while tube V1415 will conduct andenergize coil 1419. The tape drive will now be stop ed because of theconcurrent energization of coils 1417 and 1419 (see Fig. 14b).

When the end-stop senser 1500 has functioned in the described manner tohalt the ta e after it has been run through in a forward direction, itwill be noted that any tape forward signal is absolutely blocked due tothe consequent grounding of the screen grid of tube V1409 through contct 1404b by the described action initiated by contact 1502. The path ofthe taco reverse signal, however, is kept open to enable reversing thetape drive. Since the holding circuit for forward end-stop relay 1404,com rising contact 140461 is in the circuit with normally closedcontacts 1412a of relay 1412, a subsequent tape reversal is necessarv inorder to energize relay 1412 and interrunt such holding circuit.

Similarly, if the end-stop senser 1500 has functioned as described tohalt the tape after it has been unreeled in a reverse direction, a tapereverse signal will be ineffectual due to the rounding of the screen oftube 1410 through contact 1405b by the described action initiated byend-stop contact 1503. In this case, the path for the tape forward"signal is kept open to enable subsequent reversal of the tape drive in aforward direction. The holding circuit for reverse end-stop relay 1405comprises contact 14050, which is in series with normally closedcontacts 1411a of relay 1411, and a subsequent reversal of the tapedrive in a forward direction is required in order to energize relay 1411and interrupt such holding circuit.

It will be apparent that the provision of a plurality of parallelinformation channels on the tape enables the storing of indepedentunrelated information in each of the channels. Since the storedinformation cannot be read back in reverse, to obtain access to desiredstored information the tape must be reversely driven to the beginning ofthe stored area on the tape. If such stored information were neededrepeatedly, valuable time would be expended in reversing the tape aftereach reference.

In order to overcome such deficiency, the present invention contemplatesthe recording of information in op posite directions for alternatechannels. Preferably, every odd channel is employed for storing andreading information with the tape running in a forward direction, whileeach interposed even channel is utilized for storing andreadinginformation with the tape moving in a reverse direction.

I' In this manner, in order to expedite information access time,-selected information is divided up and stored in two adjacent channels.The first half of the data is stored, for example, in an odd channel,the tape moving in a forward direction. At the end of such storingoperation, the remainder of the information is stored in the adjacenteven channel, the tape moving in a reverse direction. Since storage isalways under control of the sprocket or synchronizing channel, thestored information is, in effect, inscribed in a folded loop on the tapeso that at the endof all subsequent reference to the data, the tape isalways back at the beginning and data is ready to be read again.

While a particular embodiment has been disclosed as required by thepatent statutes, the invention is not necessarily so limited, since thefeatures of instruction and formula of operation are susceptible ofvarious modes of application, limited only to the extent outlined in theappended claims.

What is claimed is:

1. In a multichannel magnetic data-recording system of the typeemploying a movable magnetic recording medium adapted to be driven ineither one of two alternate directions or to be stopped by a capstandrive having a clutch controlled by a plurality of selectivelyenergizablc solenoids, a control circuit for determining energization ofsaid solenoids comprising a current-switching device connected to eachof said solenoids, respectively, a signal feedback connection couplingindividual pairs of said switching devices whereby conduction of oneswitching device in a pair will block conduction of the other switchingdevice, a gating circuit common to each pair of said currentswitchingdevices, each of said gating devices having means for selectivelyenergizing an individual current-switching device in each pair and meansfor selectively energizing each of the gating devices.

2. A control circuit according to claim 1 in which saidcurrent-switching device comprises a thyratron.

3. A control circuit in accordance with claim 1 in which said magneticrecording medium comprises a tape, the opposite ends of which are coatedwith an electrically conductive strip, a first pair of sensing contactsadapted to be bridged by the conductive coating on a first end of thetape, a second pair of sensing contacts adapted to be bridged by theconductive coating on the opposite end of the tape, a first relayassociated with said first pair of contacts, a second relay associatedwith said second pair of contacts, circuit means connecting said firstand said second relays to said first and second gates, respectively,whereby energization of said first relay by the bridging of said firstpair of contacts will cut otf said first gate, and energization of saidsecond relay by bridging of said second pair of contacts will cut ofi?the second of said gates.

References Cited in the file of this patent UNITED STATES PATENTS2,719,884 Reed et a1. Oct. 4, 1955

